[GeoMechanicsApplication] Support USER_MOD symbol in DLL loading

[avdg81]

Description:
Currently, Kratos checks for user_mod when loading user-defined DLLs. However, Fortran compilers (e.g. Plaxis use case) may export the symbol as USER_MOD. Since Fortran is case-insensitive, USER_MOD should also be included in the lookup on our side to ensure compatibility.
Note that User_Mod is also missing.

Make a list of accepted user_mods and also make sure they work with _ after the names (GFortran compiler).

Minor fix, but useful to include in master before Thursday.